Among renewable sources of energy, solar energy consistently shows great potential to serve as a clean and inexhaustible energy source. However, the efficiency of commercial photovoltaic (PV) panels is quite low, typically around 15-20%, and the output power of PV panels is greatly affected by environmental conditions, such as temperature and solar radiation. Partial shading is one of the main causes of losses in PV power generation systems and not only reduces the maximum output power of the shaded PV module, but causes maximum power point (MPP) voltage deviation in tandem or parallel PV strings of non-shaded cells, resulting in multiple power peaks in the power-voltage characteristics of PV modules and more complexity in PV systems.
Various inverter topologies have been proposed, or are currently used in PV systems, to increase efficiency and enhance reliability by tracking of the maximum power point of the panels, as well as reducing the switching frequency. A cascaded H-bridge multilevel inverter (CMLI) is one such inverter topology, and such an inverter topology is typically suitable for transformerless, grid-connected PV systems. Recently, the Z-source/quasi-Z-source cascade multilevel inverter (ZS-CMLI/qZS-CMLI) was developed, which inherits the advantages of traditional CMLI while overcoming issues with unequal PV panel voltages among independent modules and maintaining separate maximum power point tracking (MPPT) control for each H-bridge cell.
There are numerous pulse-width modulation (PWM) techniques for CMLIs available. These techniques can be divided into two categories: sine wave PWM (SPWM) and multilevel space vector modulation (MSVM). The SPWM-based multi-carrier PWM (MC-PWM) can be phase-shifted (PS), phase disposition (PD), phase opposition disposition (POD) or alternate phase disposition (APOD). The MSVM can be 60° coordinate transformation (DCT), reference vector decomposition (RVD), general vector (GV) or sample time staggered (STS). MC-PWM is relatively flexible. Thus, it has been generally employed in applications where the voltage levels are higher than five. However, the direct current (DC) link voltage utilization of MC-PWM is lower than that of MSVM, despite injecting third-order harmonics into modulation signals to improve utilization.
Combining the CMLI with a ZS/qZS network, an additional control freedom degree (i.e., the shoot-through duty ratio) needs to be considered for each H-bridge inverter (HBI) cell. The higher the cascaded level, the more hardware comparators are required. The MSVM technique has the advantages of ideal harmonic character and high voltage utilization, and is well suited for digital implementation for ZSI/qZSI. However, increasing cascaded levels causes the selection of space vectors and the calculation of switching time for traditional multilevel SVM to become more complicated. Although an STS-SVM technique based on traditional two-level SVM solves such issues to some extent, it demands higher capacity on the hardware storage due to the staggered sampling time. Thus, for the MC-SPWM and STS-SVM, a left and right bridge vector (LRBV) based MSVM was proposed.
Thus far, the only existing PWM technique for ZS-CMLI/qZS-CMLI is PS-SPWM, and it has been studied only in simulation. MSVM has never before been applied to ZS-CMLI/qZS-CMLI. It would be desirable to provide a multilevel SVM for qZS-CMLI, which can independently insert the shoot-through for each HBI cell, thus respectively compensating the unequal PV panel voltages with high voltage utilization and low harmonics.
FIG. 2 illustrates the topology of a typical n-layer three-phase qZS-CMLI based PV power generation system. In the topology of a qZS-CMLI shown in FIG. 2, each H-bridge module contains a quasi-Z-source network, and four power switches. The modulation technique of ZS/qZS-CMLI is typically required to get on-off signals of the four switches per module, for example.
As shown in FIG. 2, each cell is composed of the qZS-based HBI (A1, A2, . . . , An) with separate PV panels (B1, B2, . . . , Bn) and (C1, C2, . . . , Cn) as DC sources. Therefore, the qZS-CMLI has the characteristics of both qZSI and CMLI, such that {circumflex over (v)}DCxi=(1/(1−2Dxi)) vPVxi=BxivPVxi; VC1xi=[(1−Dxi)/(1−2Dxi)] vPVxi and VC2xi=[Dxi/(1−2Dxi)] vPVxi; and vxn=vHx1+vHx2+ . . . +vHxn, where iϵ{1, 2, . . . , n} is the cascaded number, xϵ{a, b, c} represents the phase, vPVxi is the output voltage of each PV module, Dxi and Bxi represent the shoot-through duty ratio and boost factor per cell, VC1xi and VC2xi are the capacitor voltages, vDCxi (in FIG. 2) is the DC-link voltage of each qZS-HBI and {circumflex over (v)}DCxi is its peak value, vHxi is the H-bridge output voltage, and vxn is the phase voltage of the qZS-CMLI.
Additionally, in qZS-CMI control, traditional pulse width modulation (PWM) compares a carrier, such as the commonly used triangle or sawtooth wave, with a desired modulation signal, such as sinusoidal wave. When the modulation signal is higher than the carrier, a high signal, denoted as “1”, is generated. Conversely, for a low signal, a “0” is generated. The “1” and “0” are the signals to switch the power devices on and off. In this case, only the pulse width is modulated, and the pulse amplitude is held constant, since the ratio of modulation signal Vm divided by the carrier Vc (i.e., Vm/Vc) is invariable. Thus, traditional PWM methods result in inefficiencies, such as excessive switching losses and the like.
Further, the conventional cascade multilevel inverter (CMI) presents attractive features for photovoltaic power generation, such as high-quality step-like output voltage waveforms with lower harmonic distortions, lower requirement of power semiconductors, modular topology, etc. However, it lacks a voltage boost function, and shoot-through, i.e., a conduction phase of a switch during its transition between states, is inevitable because of the nature of the H-bridge topology. Photovoltaic systems are a problem for conventional power inverters due to the potentially large variations in the input power voltage. This presents inefficiencies that have yet to be overcome. An effective modulation method for a qZS-CMI photovoltaic power system that will maximize power efficiency from the cascaded inverters to synchronize with the grid is obviously desirable.
Thus, modulation and grid-tie control methods for quasi-Z-source cascade multilevel inverters addressing the aforementioned problems are desired.